1. Field of the Invention
The present invention relates to a dry etching method for etching a compound semiconductor in the process of manufacturing a semiconductor device, particularly, to a dry etching method involving an ion beam irradiation.
2. Description of the Related Art
Recently, compound semiconductors such as III-V group compound semiconductors and II-VI group compound semiconductors are widely used in the manufacture of optical devices such as a semiconductor laser, a light emitting diode and a photodetector. The compound semiconductors, which exhibit a high mobility, are also used widely in the manufacture of electronic devices such as a field effect transistor and a hetero bipolar transistor. A dry etching method is employed for forming a fine structure in an optional portion of such a device.
To be more specific, the dry etching method is employed for forming, for example, a laser edge surface for integrating a semiconductor laser, for forming a groove for isolating each other a plurality of elements formed on a single substrate, and for forming a fine gate. When the dry etching method is employed for such a purpose, it is necessary for the working surface and the edge surface to be flat and for the shape of the edge surface to be controlled. Particularly, the light beam emitted from an active layer is taken out from the laser edge surface. It follows that the laser edge surface is required to be as flat and as vertical (perpendicular to the laser beam) as the cleavage surface in order to prevent a loss of light caused by the decrease in the reflectance at the edge surface.
It was customary to employ a reactive ion etching (RIE) or a reactive ion beam etching (RIBE) as the dry etching method. In RIE, the etching is carried out under a low vacuum, i.e., relatively high pressure. Thus, the ion beam fails to run precisely vertically. In other words, the ion beams are uneven, making it impossible to achieve a completely vertical processing. Such being the situation, RIBE which permits the etching treatment under a relatively high vacuum is widely employed for the so-called "vertical treatment".
However, the dry etching method utilizing RIBE gives rise to a serious problem. Specifically, a chemical reaction between reactive particles and the substrate and a physical sputtering achieved by the ions are involved in the etching performed by RIBE. The chemical reaction affects the flatness of the working surface, with the vertical of the working surface affected by the physical sputtering. However, RIBE is incapable of fully satisfying both the flatness and the vertical of the working surface. For example, it is necessary to sufficiently increase the partial pressure of the reactive gas in order to achieve a sufficient flatness of the working surface by RIBE. However, the vertical of the working surface is impaired, if the partial pressure noted above is unduly high.
It is also known to the art that the angle made between the substrate surface and the direction of the reactive beam is intentionally deviated from 90.degree. so as to make the working surface, which should not be vertical originally, perpendicular to the substrate surface. In this technique, however, another surface fails to be perpendicular to the substrate surface, with the result that application of this technique in the manufacture an actual device is much restricted.
FIG. 1 shows a dry etching apparatus, e.g., a reactive ion beam etching (RIBE) apparatus. As seen from the drawing, the apparatus comprises a microwave guide 10, an etching gas inlet port 11, a plasma generating section 12, a coil 13 for generating a magnetic field, an ion extraction electrode 14, an etching chamber 15, a workpiece 19, a sample holder 16 on which the workpiece 19 is disposed, and a gas exhaust port 17. A reactive gas for the etching and an inert gas are supplied to the gas inlet port 11 through mass flow controllers 18. In the conventional etching method, the workpiece 19 is disposed first on the sample holder 16, followed by evacuating the etching chamber 15 using a vacuum pump. Then, the reactive gas is introduced into the plasma generating section through the mass flow controller 18 and the gas inlet port 11. It is possible to control the reactive gas pressure by controlling the gas flow rate through the mass flow controller 18. A microwave and a magnetic field are applied to the plasma generating section 12 so as to generate a plasma consisting essentially of ions, neutral radicals, electrons, and neutral molecules. The ions are extracted from the plasma generating section 12 by the ion extraction electrode 14 for irradiating the workpiece with said ions. In this step, a chemical reaction takes place between the workpiece and the reactive neutral radicals or neutral molecules. Also, the workpiece is sputtered by the ion bombardment so as to remove the product formed by the chemical reaction and, thus, to achieve an anisotropic etching. In the dry etching method utilizing a plasma, ion bombardment to the workpiece takes place so as to elevate the temperature of the workpiece to exceed a predetermined level during the etching treatment. Also, it takes a longer time for the temperature of the workpiece to be elevated to a predetermined level. Under the circumstances, the etching amount is not proportional to the etching time, making it difficult to control the etching amount by controlling the etching time.
In the dry etching method utilizing a plasma, a chemical reaction takes place between the workpiece and the neutral radicals or neutral molecules of the reactive gas. Thus, the etching rate is greatly dependent on the temperature of the workpiece. To be more specific, the etching rate is changed by the change in temperature of the workpiece caused by the ion irradiation during the etching treatment, giving rise to the problem that it is difficult to control the etching amount accurately as desired. To overcome this difficulty, it was proposed in the past to dispose a cooling mechanism or a heating mechanism on the sample holder in an attempt to control the temperature elevation of the workpiece during the etching treatment. However, temperature elevation of the workpiece was unavoidable during the etching treatment, making it difficult to control the etching treatment as desired.
For example, in the reactive ion beam etching (RIBE) voltage is applied across the electrodes so as to permit the generated electric field to extract the ions and excited particles under a controlled state from the plasma chamber onto the sample. In this step, the sample surface is contaminated by being sputtered the electrode. It is impossible to overcome the contamination problem, even if the electrode is made of a material which is unlikely to be sputtered such as molybdenum. It should be noted in particular that the extraction holes of the electrode are gradually enlarged by the sputtering of the electrode material, making it difficult to control the etching treatment as desired. For overcoming the difficulty so as to flatten the working surface, it is necessary to increase sufficiently the partial pressure of the reactive gas so as to remove the contaminant from the sample surface. If the partial pressure of the reactive gas is unduly increased, however, the vertical of etching is impaired. In the reactive ion etching (RIE), a plasma is generated within a sample chamber. When it comes to an apparatus in which a plasma is generated between a pair of electrodes, the apparatus is markedly contaminated with the sputtered material of the electrode material, with the result that the resultant element is adversely affected by the contamination.
As described above, it is difficult to etch a compound semiconductor by the conventional dry etching method such that the etched surface is perpendicular to an optional plane, and that the working surface and the working edge surface are flat. It is also difficult to prevent contamination derived from the peripheral parts such as the electrode.